Endohedral fullerites: A new class of ferroelectric materials

Abstract

Endohedral fullerites, composed of periodic lattices of endohedral complexes with polar molecules trapped inside the Ceo cluster, constitute a new class of ferroelectric materials and the first practical realization of ideal electric dipolar lattices. In these substances, interactions between the partially screened and freely rotating dipole moments of the guest molecules give rise to dipole-ordered low-temperature phases. The respective transition temperatures for the endohedral fullerites with LiF, LiCl, NaF, and NaCl as the guest molecules are estimated to be at 25, 36, 51, and 60 K, 64.60.Cn, 65.4aHq The known ferroelectric materials are traditionally divided into two classes that reflect the mechanisms responsible for the transitions to the low-temperature phases [1]. The first class encompasses many ionic crystals in which the individual ions move in potentials with several closely spaced minima. In these displacive-type ferroelectrics, of which barium titanate BaTiOa is a well-known example, the high-temperature phase is associated with the vibrationally averaged positions of ions, whereas the low-temperature (anti)ferroelectric phase corresponds to ions at the individual minima of the potential energy hypersurface. On the other hand, in the order-disorder-type ferroelectrics, such as potassium dihydrogen phosphate KH2PO4, the appearance of the low-temperature phase is directly linked to an order-disorder transition that often involves infinite networks of hydrogen bonds.In this Letter, we predict the existence of a new class of ferroelectric materials composed of the endohedral complexes [2] in which polar molecules are trapped inside the Ceo cluster. Although macroscopic quantities of these endohedral fullerites [3] have yet to be isolated, there is a plethora of both direct and indirect experimental evidence that confirms their existence. In particular, endohedral complexes with one [4,5] or two [6] noble gas atoms (He, Ne, and Ar) as the guests have been observed in collisional insertion experiments. Complexes with metals such as La, Ni, Na, K, Rb, and Cs [7] have also been detected among the products of laser or arc ablation of graphite rods impregnated with metal salts. Even more notably, in a very recent publication, endohedral complexes (primarily of the C60 cluster) with small polar molecules (including CaCl, CeCl, NaCl, and Nal) have been reported [8].Previous calculations [2(b)] demonstrated the absence of any significant (the activation energy of ca. 20 cal/ mol) barriers to rotation for sufficiently small guest molecules inside the Ceo cage. This observation, together with the fact that the cage screens the dipole moments of the guest molecules only partially [2,9], means that the endohedral fullerites can be regarded as the first practical realization of ideal electric dipolar lattices. The dipolar lattices have been the subject of several theoretical stud-

Relação entre transferência de carga e as interações intermoleculares em complexos de hidrogênio heterocíclicos

“…De acordo com a implementação feita por Cioslowski e colaboradores [51][52][53] , os cálculos AIM também foram processados utilizando o programa Gaussian 98W. A energia da ligação de hidrogênio foi determinada de acordo com a aproximação da supermolécula 13 conforme a Equação 8…”

Rovibrational Motion of CO in SolidC60

“…Small molecules inside fullerene cages have been suggested to form an ideal system for the study of nearly free molecular rotation in the solid, and experimental evidence for the motion of a scandium trimer inside a C 82 fullerene cage has, for instance, been reported [16]. In addition, it has been pointed out that small polar molecules trapped inside the highly symmetric C 60 cage might be considered the building blocks for an ideal electric dipolar lattice [17]. The results reported here indicate that samples in which small molecules are trapped in the octahedral sites of solid C 60 , i.e., in between C 60 molecules, samples that are much easier to produce, are equally interesting and promising and might show many of the characteristics, and enable many of the applications, anticipated for the corresponding endohedral species.…”

Spectroscopy of light-molecule endofullerenes

“…Interesting electrical properties have been postulated for endofullerenes in which the molecular guest possesses an electric dipole moment [61]. A solid crystal of such a substance comprises a regular array of freely rotating electric dipoles, which interact mutually through electrostatic forces.…”